As electronic devices increasingly become portable, advances must be made in energy storage systems to enable such portability. Indeed, it is often the case with current electronic technology that the limiting factor to portability of a given device is the size and the weight of the associated energy storage device. A small energy storage device, such as a battery, may be fabricated for a given electrical device but at the cost of energy capacity. Conversely, a long lasting energy source can be built but it is often too large or too bulky to be comfortably portable. The result is that the energy source is either too heavy or does not last long enough for a particular user's application.
Numerous different battery systems have been proposed for use over the years. Early rechargeable battery systems included lead acid, and nickel cadmium (NiCad), each of which has enjoyed considerable success in the market place. Lead acid batteries are preferred for applications in which ruggedness and durability are required and hence have been the choice of automotive and heavy industrial settings. Conversely, NiCad batteries have been preferred for smaller portable applications. More recently, nickel metal hydride systems (NiMH) have found increasing acceptance for both large and small applications.
Notwithstanding the success of the foregoing battery systems, other new batteries are appearing on the horizon which offer the promise of better capacity, better power density, longer cycle life, and lower weight, as compared with the current state of the art. The first such system to reach the market is the lithium ion battery, which is already finding its way into numerous consumer products. Lithium polymer batteries are also receiving considerable attention, although they have not yet reached the market.
Lithium ion batteries in general include a positive electrode fabricated of, for example, a transition metal oxide material and a negative electrode fabricated of an activated carbon material such as graphite or petroleum coke. New materials for both electrodes have been investigated intensely because of the high potential for improved energy density. Typically the positive and negative electrodes are permeated by a shared electrolyte medium, and are held in close proximity at a uniform distance from each other so as to minimize cell polarization while maximizing the uniformity and efficiency of capacity utilization across the cell. To prevent short circuits and yet allow ion migration across the cell, a thin plastic microporous membrane is commonly placed between the negative and positive electrodes of lithium ion cells.
The pressure exerted to maintain the cell in the preferred dimensions is referred to as "stack pressure" due to the serial arrangement of cells either as flat stacks or as a single cell wound as an evenly spooled coils around itself in a circular "stack". In commercial lithium ion cells, stack pressure is typically enforced by placing a tightly rolled ("jelly roll") cell into a rigid metal can (often cylindrical in shape) with internal dimensions that are only slightly larger than the full size of the "jelly roll". However recently the industry has been moving toward lighter packaging materials: when the cell is housed instead in a flexible, heat-sealable material, such as a thin foil bonded in a sandwich to layers of plastic sheeting, the housing material can no longer be expected to provide sufficient rigidity and strength to enforce the stack pressure of the cell.
Accordingly, there exists a need for improved cell configurations to constrain cells to preferred dimensions and to constrain opposite electrodes to close, uniform proximities.